Rotary tables, particularly those used in the machine-tool industry are typically constructed using a rotary plate, pivotably mounted to a base with some mechanical means for moving the plate relative to the base. A wide variety of mechanical mechanisms having been used for rotating a plate relative to the base such as a gear and pinion or worm screw and ring gear. In other applications where close tolerance is not necessary, or where the table location need not be infinitely adjustable, cam and Geneva mechanisms have been employed.
Various rotary tables have been made in the past using a fluid-powered unit, such as a hydraulic cylinder, as a drive means. Rotary tables have been made using a rack gear which drives a rotating pinion coupled to the table, where the rack is moved linearly by conventional hydraulic cylinder. Hydraulic cylinders having a rack integrally formed in the pistonrod are manufactured by Ohio Oscillator, Inc. and are commercially available. Another prior art method of using a standard hydraulic cylinder to power a rotary table is shown in U.S. Pat. No. 3,825,245 (Osborne). The Osborne patent discloses a pallet changer which uses a standard hydraulic cylinder which is attached at one end to the base and at the other end to the rotary plate at a point spaced from the centerline of rotation so that the linear movement of the cylinder causes rotation of the base relative to the plate. The Osborne device is designed to rotate 180 degrees quickly and accurately.
The principal disadvantage of the gear-driven mechanical tables is that they are slow and very expensive to manufacture if accuracy is desired. The cam or Geneva drive mechanical rotary tables operate faster and are somewhat less expensive, however, it is difficult to accurately control relative motion and the mechanism is frequently bulky. Hydraulic systems having a linear cylinder are likewise bulky and the travel is limited by the cylinder length.